1. Field of the Invention
The present invention relates to an organic light emitting display and its method of fabrication, and more particularly, the present invention relates to a top-emission organic light emitting display in which a cathode electrode has a reduced resistance and its method of fabrication.
2. Description of the Related Art
Generally, a light emitting display is a self-luminous display which emits light by electrically exciting fluorescent compounds. The light emitting display has characteristics of low voltage driving, facilitation of slimness, a wide viewing angle, and a fast response speed, which are less favorable in a liquid crystal display, so that it has been spotlighted as a next generation display.
The light emitting display may be divided into inorganic light emitting displays and organic light emitting displays according to whether materials forming a light emitting layer are inorganic matter or organic matter.
An organic light emitting display includes an organic layer with a predetermined pattern formed on a glass substrate or other transparent insulating substrate and anode and cathode electrode layers formed on the upper and lower faces of the organic layer. The organic layer is formed of organic compounds including a light emitting layer.
In the organic light emitting display formed as above, holes injected from the anode electrode supplied with a positive voltage move to the light emitting layer via a hole transportation layer, and electrons are injected from the cathode electrode supplied with a negative voltage via an electron transportation layer into the light emitting layer, when positive and negative voltages are supplied to the anode and cathode electrodes, respectively. Thus, the electrons and the holes are recombined in the light emitting layer to generate exitons, which are changed to a ground state from an excited state. Accordingly, images are formed by fluorescent molecules in the light emitting layer emitting light.
The organic light emitting display may be divided into top-emission and bottom-emission types according to a light emitting direction. Recently, the top-emission type of organic light emitting display has been preferably adopted for large flat displays.
In the case of the top-emission type of organic light emitting display, the anode electrode is formed on the lower portion of the organic layer and the cathode electrode is formed on the upper portion of the organic layer where light is transmitted.
Also, the anode electrode, which is a reflection electrode, is formed of a metal having a high reflectivity, such as Ag, Al, etc. and uses a resonance structure to achieve color reproducibility through high purity color implementation and high efficiency by resonance of light. In the resonance structure, the cathode electrode uses a semi-transparent layer. In the of the top-emission type of light emitting scheme, the cathode electrode having a low work function and semi-transmitting characteristic is needed.
In order to achieve the semi-transmitting characteristic, the cathode electrode needs to be formed thinly. In this case, it is a disadvantage since it results in the cathode electrode having a high resistance.
When operating the organic light emitting display with a current drive, a voltage drop (IR drop) occurs due to the resistance of a line and/or electrode. In particular, in the case of a top-emission type of organic light emitting display having a thin cathode electrode, a voltage drop problem becomes serious due to the high resistance of the cathode electrode, so that the display cannot perform well. In other words, when the cathode electrode has a high resistance, non-uniformity of luminance is severe inside a panel.